Not every AS- and ASD-related website, just the ones that are exceptionally reliable and helpful. This list will be growing weekly, so stop back often.
Autism Spectrum Disorders Generally
Association for Science in Autism Treatment (ASAT) should be your first stop: http://www.asatonline.org/
Autism Information Center, The Centers for Disease Control: http://www.cdc.gov/ncbddd/autism/
Autism Speaks: http://www.autismspeaks.org/
National Institute of Mental Health publication, "Autism Spectrum Disorders (Pervasive Developmental Disorders)": http://www.nimh.nih.gov/health/publications/autism/complete-index.shtml
Medline Plus, "Autism" (includes dozens of good links): http://www.nlm.nih.gov/medlineplus/autism.html
Asperger Syndrome
Online Asperger Syndrome Information and Support/OASIS: http://www.aspergersyndrome.org
Tony Attwood's Website: http://www.tonyattwood.com.au
Applied Behavior Analysis/ABA
Behavior Analyst Certification Board (BACB), includes searchable database/listing of Board Certified Behavior Analysts throughout the world: http://www.bacb.com/
Journal of Applied Behavior Analysis (JABA). Fantastic peer-reviewed journal; all articles available to download at no charge: http://seab.envmed.rochester.edu/jaba/
Cambridge Center for Behavioral Studies: http://www.behavior.org/autism/
Precision Teaching Hub: http://precisionteaching.pbwiki.com/FAQs--about+the+Standard+Celeration+Chart
Fluency.org, comprehensive links on precision teaching, fluency, and the standard celeration chart: http://www.fluency.org/
Christina Burke, MA, great, clear explanations of all things ABA: http://www.christinaburkaba.com/
Dr. Vincent Carbone, the Carbone Clinic, lots of great information on Verbal Behavior: http://www.drcarbone.net/.
Dr. Jack Michael, papers, PowerPoints, amazing stuff: http://jackmichael.org/
Dr. Mark Sundberg, downloads, PowerPoints, ABA and Verbal Behavior: http://www.marksundberg.com/
Dr. James Partington, source for the ABLLS-R (Assessment of Basic Language and Learning Skills--Revised) and information on ABA: http://www.behavioranalysts.com/
Related Disorders
Tourette Syndrome
National Tourette Syndrome Association: http://www.tsa-usa.org/
Tourette Syndrome "Plus" (one of the best websites): http://www.tourettesyndrome.net/
National Institute of Neurological Disorders and Stroke / National Institutes of Health: http://www.ninds.nih.gov/disorders/tourette/detail_tourette.htm
Attention Deficit/Hyperactivity Disorder
CHADD, Children and Adults with Attention Deficit/Hyperactive Disorder: http://www.chadd.org/
National Institute of Mental Health publication, "Attention Deficit Hyperactivity Disorder" publication: Autism Spectrum Disorders (Pervasive Developmental Disorders)": http://www.nimh.nih.gov/health/publications/autism/complete-index.shtml
Anxiety Disorders, including OCD
Obsessive Compulsive Disorder, "OCD in Children": http://www.ocfoundation.org/ocd-in-children.html
WorryWiseKids.org, Dr.Tamar Chansky cofounded this organization; great information on anxiety, OCD, selective mutism, PANDAS, phobias, posttraumatic stress disorder, and all related disorders: http://www.worrywisekids.org/index.html